This disclosure relates to differential output buffers that output emphasized differential output signal in which transitions in the differential data signal are emphasized.
Communication systems that transmit and receive differential data signals are widely used. A pre-emphasis technique is used in transmitting and receiving differential data signals in order to compensate attenuations of high-frequency signals during the transmission in wirings or the like. The pre-emphasis is a technique to emphasize the amplitude of signals at rising and falling edges at the transmitting side in order to compensate attenuation of high-frequency signals at the receiving side.
In the pre-emphasis technique, it is required to variably adjust the emphasis amount at the rising and falling edges depending on the characteristics of the transmission paths. It is also required to emphasize the signal in multiple steps in time with varying levels. For example, the emphasis amount may be maximized immediately after the rising and falling edges, slightly decreased thereafter, and then further decreased. For example, 10G-K specification for transmitting up to 1 meter length on printed circuit boards, which is one of IEEE 802.3 communication specifications, requires the emphasis in up to three steps.
US Patent Application No. 2008-0218222 (Patent document 1) proposes a technique to vary the emphasis level in multiple steps and to continue the emphasis in multiple steps in time with varying levels. Patent document 1 discloses a differential output buffer including a pre-driver stage and an output driver stage. The pre-driver stage includes a first differential pair that performs a current subtraction. The output driver stage includes a second differential pair and receives a second differential data signal from the pre-driver stage and outputs an output signal to a transmission line.
The pre-driver stage can generate signals that accurately represent the emphasis levels. The output driver stage receives differential signals generated by the pre-driver stage at the gates of transistors of the second differential pair. However, the output driver stage cannot always generate accurately emphasized output signals. That is, the output driver stage has a relatively high gain when the amplitude of the differential signal input to the gates of the transistors in the second differential pair is small. When the amplitude of the input differential signal increases, however, the output driver stage saturates and its gain decreases. As a result, the amplitude of the output signal cannot be accurately controlled by the amplitude of the signal input from the pre-driver stage. Accordingly, it is impossible to realize emphasis levels represented by the signals generated by the pre-driver stage.
As explained above, the emphasis technique requires a function to control the emphasis amounts in multiple levels and also requires an accurate control of the emphasis levels.